1. Field of the Invention
The present invention relates to an improvement of an electric connection terminal for a sensor element using ceramics, such as an oxygen sensor or the like which can be used for detecting an oxygen concentration in an exhaust gas of an automobile engine.
2. Related Art Statement
Heretofore, as an oxygen sensor for detecting an oxygen concentration in a substance to be detected such as an exhaust gas of an automobile engine or the like, a sensor has been proposed of a structure wherein each electrode layer is provided on both surfaces of a distal end of an elongated flat sensor substrate late consisting mainly of zirconia, one electrode being exposed to a standard substance such as air, the other electrode being exposed to a substance to be detected such as an exhaust gas, and an electromotive force produced between the two electrodes by the principle of an oxygen concentration cell is used as a detected signal.
This type of oxygen sensor has generally, a connection terminal for connecting a plurality of covered lead wires at the rear end of the sensor substrate, and the covered lead wires connected to the connection terminal for transmitting the detected signal to the outer circuit as an output.
Soldering of the covered lead wires one by one to the connection terminal of the sensor substrate is not efficient. Even when the covered lead wires are connected by a suitable caulking element to auxiliary lead wires extending from the connection terminal, the work of caulking is difficult and not efficient. Therefore, in order to solve the problem of electric connection of the sensor element with the covered lead wires, the inventors have previously proposed an oxygen sensor as Japanese Utility Model Applicaton Laid-open No. 150,449/85, which was laid-open to the public on Oct. 5, 1985, which is after the priority date of the above-identified application (Japanese Utility Model Application No. 38,406/84, U.S. patent application Ser. No. 709,802, now U.S. Pat. No. 4,588,494. FIG. 5 of the attached drawings shows a longitudinal cross-sectional view of the whole structure of the oxygen sensor with reference numeral 40.
Explaining FIG. 5 in detail, a sensor element 41 composed of an oxygen ion conductive solid electrolyte plate and two electrodes, etc., is accommodated in a cylindrical metallic protective tube 42 in such a fashion that the distal end 44 provided with the electrodes becomes the lower portion of the sensor element 41. The intermediate portion of the sensor element 41 is supported by an insulating porcelain 43. The upper end of the sensor element 41 is supported by a connector porcelain 45. The sensor element 41 is fixed in the protective tube 42 by means of cement, talc, glass or the like filler 46 filled above and below the insulating porcelain 43.
The protective tube 42 has many perforation holes 47 at its lower end so that the measuring electrode arranged at the outer surface of the lower end of the sensor element 41 comes into contact with a gas atmosphere to be detected.
The protective tube 42 has at its upper end an earth lead wire 50 contacting the inner wall of the protective tube 42 and lead wires 51-53 inserted through a rubber stopper 48. The connector insulator 45 assumes a cylindrical shape, and has as its lower portion an insertion hole 55 for inserting the upper end of the sensor element 41, and at its upper portion lead wire insertion holes 56-59 for inserting the lead wires 50-53 which communicate with the insertion hole 55, as shown in the attached FIGS. 7 and 8. The lead wires 50-53, which are inserted into the lead wire insertion holes 56-59, are fixed at their ends by means of caulking metallic fittings 62, 66 which are integrally formed with folded resilient contacting elements 61, 65, as shown in the attached FIG. 6.
The contacting elements 61, 65 contact under pressure with the sensor element 41 which is inserted into the insertion hole 55 of the connector porcelain 45, so that they contact with connection terminals 60, 64 arranged at the surfaces of the end of the sensor element 41.
The lead wires are four in number in the illustrated structure, because the oxygen sensor 40 is provided with a heater so that the heater is arranged integrally with the sensor element 41 and two lead wires are necessary for supplying electric current to the heater from an electric power source.
In this way, the connector porcelain 45 functions as a connector socket, so that the connection between the lead wires 50-53 and the connection terminals 60, 64 of the sensor element 41 is facilitated.
However, the above connection terminals 60, 64 arranged on the sensor element 41 have hitherto been fixed on the sensor element 41 by baking a film of a metal such as nickel, gold, silver or platinum, etc. on surfaces of a plane plate of a solid electrolyte, so that they have a drawback in that the bonding force of the terminals to the sensor element is not strong. For instance, when the oxygen sensor 40 is mounted on an automobile, there is a drawback in that the connection terminals 60, 64 are peeled off from the sensor element 41 by friction thereof against the contacting elements 61, 65 due to vibration of the automobile and repetition of heating and cooling of the sensor element 41.